Creating the function MessageCache::getExtensionMessagesFor, to get an array of the...

Creating the function MessageCache::getExtensionMessagesFor, to get an array of the extension messages in a specific language, and avoid the ugly hack in Special:Allmessages, which uses a private variable of wgMessageCache and uses is_array to check the array of messages, which includes both regular strings (as should be) and arrays of the languages.